Flexible duct with shrinkage-proof film

ABSTRACT

The invention concerns a flexible duct ( 1 ) comprising from inside outwards a carcass ( 2 ) produced by a short-pitched metallic helical winding forming butt gaps ( 12 ), an overlapping layer ( 10 ) acting as a heat shield consisting of helically wound plastic strips ( 10   a   , 10   b ), a sealed inner pressure sheath ( 3 ), extruded in plastic on said overlapping layer ( 10 ), at least an armouring web ( 5 ) and at least an outer sealing sheath ( 7 ). The invention is characterised in that the overlapping strips ( 10   a   , 10   b ) are wound at a winding angle less than 35°.

[0001] The present invention relates to a flexible pipe that can be usedfor transporting fluids, such as hydrocarbons for example.

[0002] Several types of flexible pipe are used. Some flexible pipescomprise, from the inside outward, an internal sealing sheath made of aplastic, an elastomer or another relatively flexible suitable material;an unsealed flexible metal tube that has to withstand the forcesdeveloped by the pressure of the fluid flowing in the pipe; one or morearmor plies and at least one external sealing sheath made of a polymericmaterial. This type of flexible pipe is often called a smooth-bore pipeby experts in the field.

[0003] Other flexible pipes, called rough-bore pipes, comprise, from theinside outward, an unsealed flexible metal tube, called a carcass,formed by a profile wound in turns and mutually interlocked, such as,for example, an interlocked strip or an interlocked shaped wire such asa T-shaped, U-shaped, S-shaped or zeta-shaped wire; an internal sealingsheath made of a polymeric material; one or more armor plies capable ofwithstanding the forces developed by the pressure of the fluid flowingin the pipe and the external forces to which the flexible pipe issubjected; and at least one external protected sheath of the polymerictype.

[0004] In the latter type of flexible pipe, the internal sealing sheathis extruded, continuously, directly over said carcass, which hasinterstices or gaps between the wound turns.

[0005] To ensure good contact between the internal sealing sheath andthe metal carcass, it is necessary for the inside diameter of theinternal sealing sheath to be as close as possible and even equal to theoutside diameter of the flexible metal carcass.

[0006] During manufacture of a rough-bore flexible pipe, the internalsealing sheath, which is extruded over the metal carcass, contracts ontothe latter during cooling. Depending on the materials used for producingthe internal sealing sheath, after cooling deformations called“shrinkage cavities” are observed, these cavities appearing on theinternal face of said internal sealing sheath and especially on eitherside of the gaps between the turns of the metal carcass. Such shrinkagecavities are due, it would seem, to the differential shrinkage of thematerial used for the internal sealing sheath, because of the variationin the cooling gradient through the thickness of the internal sealingsheath, combined with the effect of the gaps between the turns of themetal carcass. Since the extruded plastic sealing sheath is in contactby its internal face with the metal carcass, which is at roomtemperature, this results in said internal face cooling very rapidly,thereby causing surface irregularities or shrinkage cavities; thisphenomenon is exacerbated at the gaps between the turns of the metalcarcass, the differential shrinkage at these points causing localvariations in the thickness of the internal sealing sheath. When thesealing sheath is made of a semicrystalline polymer sensitive to thepresence of surface defects causing a weakening of the sheath, possiblyto the point of failure, such as PVDF (polyvinylidine fluoride) forexample, this very often leads, in operation, to degradation (failure)of said sealing sheath, which then no longer fulfils its sealingfunction.

[0007] To remedy such a drawback and to solve the problem posed by theappearance of shrinkage cavities, a first solution consisted in placing,between the metal carcass and the internal sealing sheath, a thinsacrificial underlayer (thickness about 2 to 3 mm) made of a suitablematerial such as PVDF, which then serves as a heat shield. The internalsealing sheath is extruded over said sacrificial underlayer, but withoutany assurance that there is intimate bonding or “welding” between thesealing sheath and the sacrificial underlayer, so that cracks, that canpropagate from the internal face of the underlayer to the outside, areblocked at the sealing sheath/sacrificial underlayer interface.

[0008] The major drawback of this solution is the slip that is liable tooccur between the internal sealing sheath and the sacrificial underlayerat the ends of the flexible pipe, and the additional raw material andconversion costs incurred by the presence of said sacrificialunderlayer.

[0009] Provision could be made to extrude a thinner sacrificial sheath(thickness less than or equal to 1 mm), but, because of the diameter ofthe extruded tube (greater than 10 cm), it is impossible for so thin atube to be extruded on an industrial scale. It is therefore limited to a2 to 3 mm thick sheath. In addition, the operation requires theintermediate sheath to be wound on an intermediate reel and, since theintermediate sheath is thin, it will buckle during winding.

[0010] To avoid these drawbacks, document FR 2 752 904 (COFLEXIP)proposed a process for manufacturing flexible pipes that consists inheating the flexible metal tube or metal carcass to a temperature ofbelow 100° C., upstream of the extrusion means, so as in this way toavoid suddenly cooling the internal face during extrusion over the metalcarcass.

[0011] For plastics of very low viscosity, it is necessary to heat theproduct to a very high temperature, and consequently to heat the carcassto a high temperature. This high temperature induces very substantialthermal creep in the gaps of the carcass, requiring the insertion, intothese gaps, of a rod that limits the volume of creep in order to preventblockage of the carcass. Such a rod is described in document FR 2 779797 (COFLEXIP), but the spiraling of the rod in the gaps of the carcassis not simple to implement. Documents EP 0 749 546 (COFLEXIP-ELFATOCHEM) and FR 2 732 441 (COFLEXIP) disclose the short-pitch helicalwinding of an intermediate strip for following and partially filling thegaps, and are therefore similar to the previous solution.

[0012] Another solution, proposed in document EP 166 385 (FURUKAWA),consists in winding, around the carcass, several layers of thin plastic(for example polyester) tapes (thickness about 0.5 mm for a tube about 2to 8 cm inside diameter). This interlayer masks the gaps and preventsthe sealing sheath from creeping into the gaps in the carcass. Thedetails of the winding are not explained in the document.

[0013] The interlayer, by preventing the sealing sheath from creepinginto the carcass, consequently also prevents the sheath from bonding tothe carcass and therefore creates problems of slip between the twolayers. For riser applications, this may cause the flexible pipe todeteriorate: since the gap between the turns is not controlled by thecreep indentations, the carcass can slip under its own weight, the gapsbetween turns being canceled out and accumulating at the base of theriser, causing destruction of the carcass in the upper part.

[0014] The objective of the invention is to propose, within the contextof a flexible pipe, a carcass-covering system that serves as a heatshield for the extruded plastic sheath so as to prevent the formation ofshrinkage cavities, but does not have the drawbacks of theaforementioned solutions and enables it to be put into place simply.

[0015] The invention achieves its objective by means of a flexible pipeof the type comprising, from the inside outward, a carcass formed by ashort-pitch helical metal winding leaving gaps, a covering layer servingas a heat shield, consisting of helically wound plastic tapes, aninternal sealed pressure sheath made of a plastic extruded over saidcovering layer, at least one armor ply wound around said sealing sheathand at least one external sealing sheath, characterized in that thecovering tapes are wound with a long pitch, that is to say with awinding angle of less than 35°.

[0016] To do this, a taping unit is placed upstream of the sheathextruder in order to wind several thin plastic tapes around the carcasswith a long pitch, preferably between 10° and 35°.

[0017] These tapes are preferably wound with an overlap, thereby makingit possible to ensure that the entire carcass is covered with thislayer. This overlapping of the tapes may create a defect on the internalsurface of the extruded sheath, but this possible defect is a long-pitchhelical defect, and therefore does not have a great influence on thesealing sheath. The overlap of the tapes is typically about 10% of thetape width.

[0018] By winding the tapes with a long pitch, the scrapers of theextruder are prevented from lifting these tapes up and the length oftapes needed for covering the entire length of the flexible pipe is thuslimited, which avoids having to recharge the taping unit duringproduction.

[0019] The tape is thick enough to serve as a heat shield for thesealing sheath during extrusion (until the internal surface of thesheath has solidified) in order to prevent the formation of shrinkagecavities, but thin enough, however, to allow indentation of the sealingsheath into the gaps in the carcass in order to allow bonding thereto.Advantageously, the indentations form approximately 20% to 75% of thevolume of the gap. The thickness of the tape also depends on thematerial of which it is made, but it is generally less than one fifth ofthe thickness of the pressure sheath and preferably about 1 mm or evenless.

[0020] Since the tape plays no part in the behavior of the flexiblepipe, it may very well degrade on contact with the extruded plasticsheath, from the moment that it nevertheless acts as a heat shield untilthe internal surface of the plastic sheath has solidified. It thereforehas to have a lifetime of approximately one minute.

[0021] To be able to install this tape in great length and to avoid anyrisk of fracture during its installation (which would require restartingthe extrusion) or any risk of uncontrolled elongation of the tape, it ispreferable for the tape to have a tensile strength of greater than 20daN (the tensile force imposed by the taping unit). This strength may beprovided by the basic constituent material of the tape or byreinforcements, especially by metal reinforcements or textilereinforcements (carbon or Kevlar®-type fibers or rovings) for example inthe form of longitudinal reinforcements or meshes.

[0022] The width of the tapes is preferably close to the outsidediameter of the carcass so as to ensure that it is covered with 3 to 5tapes.

[0023] The constituent material of the covering tapes is chosen to havea relatively low flexural strength (elastic modulus) so as to deform atthe gaps, in order to allow the desired creep when the sheath contracts.

[0024] The constituent material of the tapes may be of the reinforced orunreinforced heat-shrinkable type, which may improve the formation ofthe indentations.

[0025] The tape may have longitudinal notches (which reduce the tensilestrength of the tape), the indentation of the pressure sheath into thecarcass taking place through these notches.

[0026] The tapes must be made of a material compatible with the pressuresheath, that is to say with polyolefins, polyamides (preferably of theRILSAN® is nylon-11 type), modified or unmodified fluoropolymers(homopolymers or copolymers) (polyvinylidene fluoride PVDF orpolyfluoroalcoxy) or hydrocarbon, fluorinated or fluorosiliconelastomers (thermoplastic elastomer or thermoplastic urethane).

[0027] The tape must have a flexural strength less than the force ofcontraction of the pressure sheath as it cools down. The flexuralstrength of the tape depends on the hot modulus and the thickness of thetape. The force generated by the contraction of the pressure sheathdepends on the thickness of the pressure sheath, the hot elastic modulusof this sheath and the size of the gap. Those skilled in the art willknown how to determine the behavior of the pressure sheath as it coolsdown, for example using ABAQUS software, and consequently the maximumtape thickness for allowing indentation of the pressure sheath into thecarcass.

[0028] In the case in which the tape is eliminated on contact with theextrudate (pressure sheath), an example of a suitable material for thetape is a polyester polyurethane (TPU) known by the name ESTANE® 58271,the viscosity of which decreases rapidly at high temperature (the layermay be eliminated in 30 days at 120°). A small thickness in contact withthe extrudate (at an extrusion temperature above 230° C. in the case ofa PVDF), the tape will have a lifetime of a few minutes, sufficient toallow the internal surface of the pressure sheath to solidify(solidification temperature close to 160-170° C. in the case of a PVDF).

[0029] Other advantages and features of the present invention willbecome more clearly apparent on reading the following description,together with the appended drawings, in which:

[0030]FIG. 1 is a partial perspective view of a rough-bore flexible pipethat includes the covering tapes according to the invention;

[0031]FIG. 2 is a partial longitudinal sectional view of part of theflexible pipe of FIG. 1, showing the covering tapes and the sheath thatcovers them; and

[0032] FIGS. 3 to 5 are schematic representations of tapes suitable forthe invention.

[0033] A rough-bore flexible pipe 1 comprises, from the inside outward:

[0034] a flexible metal tube or carcass 2, formed by a short-pitchhelical winding (that is to say one with a winding angle close to 90°)of a metal strip or wire of predetermined cross section;

[0035] an internal sealing or pressure sheath 3 made of a polymer,placed around the carcass 2;

[0036] a pressure vault 4 resistant mainly to the pressure developed bythe fluid in the sealing sheath and formed by the short-pitch helicalwinding (that is to say one with a winding angle close to 90°) aroundthe internal sheath of one or more interlocked profiled metal wires(which may or may not be self-interlockable); the profiled wires have aZ-shaped or T-shaped cross section or one derived therefrom (teta orzeta), or a U-shaped or I-shaped cross section;

[0037] one or more armour plies 5 wound with a long pitch, for exampletwo crossed tensile armour plies whose lay angle, measured along thelongitudinal axis of the pipe, is less than 60°;

[0038] optionally, one or more intermediate bands 6; and

[0039] an external sealing sheath 7 made of a polymer.

[0040] According to the invention, a covering layer 10, formed bycovering tapes 10 a, 10 b, etc., wound with a long pitch and with aslight marginal overlap 11, was placed between the carcass 2 andpressure sheath 3. As shown in FIG. 2, at a gap 12 between two adjacentturns of the carcass 2 (formed here by an S-shaped self-interlockedstrip), after the sheath 3 has been extruded, the tape 10 a or 10 b isforced into the gap, forming a helical groove filled by the extrudedsheath 3 and thus forming a helical indentation 13.

[0041] The tapes forming the covering layer are wound with a long pitch,with a winding angle of less than 35°. According to an extreme case (notshown), the covering layer may be formed by one or more tapes placedlongitudinally along the axis of the pipe (the “winding” angle thenbeing 0°).

[0042] FIGS. 3 to 5 show various examples of covering tapes that can beused in the invention. The tape 10 a in FIG. 3 has longitudinalreinforcements 14, for example made of Kevlar® or made of carbon, thesebeing placed within a plastic matrix. In FIG. 4, the reinforcementsincorporated into the material of the tape 10 a are in the form of amesh 15, for example made of metal. In FIG. 5, the tape 10 a includeslongitudinal incisions or notches 16 that allow passage of theconstituent material of the sheath during extrusion of the latter andfacilitate the formation of larger indentations.

1. A flexible pipe (1) of the type comprising, from the inside outward,a carcass (2) formed by a short-pitch helical metal winding leaving gaps(12), a covering layer (10) serving as a heat shield, consisting ofhelically wound plastic tapes (10 a, 10 b), an internal sealed pressuresheath (3) made of a plastic extruded over said covering layer (10), atleast one armor ply (5) and at least one external sealing sheath (7),characterized in that the covering tapes (10 a, 10 b) are wound with awinding angle of less than 35°.
 2. The pipe as claimed in claim 1,characterized in that the winding angle of the tapes (10 a, 10 b) isbetween 10° and 35°.
 3. The pipe as claimed in either of claims 1 and 2,characterized in that the tapes (10 a, 10 b) are wound with an overlap.4. The pipe as claimed in any one of the preceding claims, characterizedin that the tapes (10 a, 10 b) have a thickness of less than or equal to1 mm.
 5. The pipe as claimed in any one of the preceding claims,characterized in that the tapes (10 a, 10 b) have a width of about theoutside diameter of the carcass (2).
 6. The pipe as claimed in any oneof the preceding claims, characterized in that the covering layer (10)is formed from 3 to 5 tapes (10 a, 10 b).
 7. The pipe as claimed in anyone of the preceding claims, characterized in that the tapes (10 a, 10b) are made of a reinforced plastic.
 8. The pipe as claimed in any oneof the preceding claims, characterized in that the tapes (10 a, 10 b)have a tensile strength greater than 20 daN.
 9. The pipe as claimed inany one of the preceding claims, characterized in that the tapes (10 a,10 b) have longitudinal notches.